--> Empirical Relationship Between Gas-Chemical Composition and Thermal Maturity in Eagle Ford Formation, South Texas

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Empirical Relationship Between Gas-Chemical Composition and Thermal Maturity in Eagle Ford Formation, South Texas

Abstract

Organic-rich and carbonate-rich Eagle Ford Formation is a self-sourced oil and gas reservoir with little alteration of gas chemistry affected by petroleum expulsion and migration, and provides an ideal natural laboratory to quantify the compositional variation of gases generated from oil-prone type II kerogen during thermal maturation. The chemical composition of the released gas from rock crushing was conducted and integrated with RockEval pyrolysis to define the empirical quantitative relationship between gas chemical compositional parameters and thermal maturity in this study. From 10 wells in the Eagle Ford shale in South Texas, we collected 74 organic-rich shale core samples having a range of thermal maturity (Ro values from 0.51% to 1.52%). TOC content varies from 0.3% to 8.53%, with an average of 3.12%; burial depth is from 2989.6 ft to 13827.3 ft. Our results showed that gas-chemical composition is mainly controlled by thermal maturity, and three stages of gas generation are identified based on the C1 and C2 concentrations of the gases released by rock crushing from Eagle Ford core samples. The three stages of gas generation correspond to the following processes of organic-matter conversion: (1) kerogen and bitumen thermal cracking to crude oil, (2) bitumen and heavy crude oil thermal cracking to light oil, and (3) secondary oil cracking to gas. CH4-rich gas and abundance of branched butane and pentane is generated in oil cracking to gas, resulting in high C1/C2, C1/(C2+C3), i-C4/n-C4, and i-C5/n-C5 ratios. Enhanced isomerization is an indication of the early oil cracking to gas. Empirical equations between gas compositional parameters and thermal maturity (Ro) are obtained for oil-prone Type II kerogen. Vitrinite reflectance (Ro) as a thermal maturity indicator is calculated based on Tmax values based on RockEval pyrolysis on the same core samples. C1, C2, C1/C2, C1/C2+C3, and i-C4/n-C4 ratios are the five best parameters with an R-squared value of 0.74. The composition of gas produced from the Eagle Ford Formation following hydraulic fracturing is used to validate the empirical equations. Calculated vitrinite reflectance dominantly ranges from 1.0 to 1.1% Ro, suggesting that the favorable thermal regime for the oil production is located at the peak of oil generation and the beginning of oil cracking to gas. Empirical equations provide a basis for interpretation of mud-gas logging data and produced gas-chemistry data.